The present invention relates to processes for producing borohydride compounds. In particular, the present invention provides efficient processes for the large-scale production of borohydride compounds.
Environmentally friendly fuels, e.g., alternative fuels to hydrocarbon based energy sources, are currently of great interest. One such fuel is borohydride, which can be used directly as an anodic fuel in a fuel cell or as a hydrogen storage medium, e.g., hydrogen can be liberated by the reaction of sodium borohydride with water, which produces sodium borate as a byproduct. As with all fuels, borohydride must be manufactured from readily available materials. Thus, there is a need for improved and energy efficient industrial scale manufacturing processes for producing borohydride compounds.
Typical industrial processes for the production of sodium borohydride are based on the Schlesinger process (Equation 1) or the Bayer process (Equation 2), which are both described below. Equation 1 illustrates the reaction of alkali metal hydrides with boric oxide, B2O3, or trimethyl borate, B(OCH3)3, at high temperatures, e.g., ca. 330xc2x0 to 350xc2x0 C. for B2O3 and 275xc2x0 C. for B(OCH3)3.
4NaH+B(OCH3)3xe2x86x923NaOCH3+NaBH4xe2x80x83xe2x80x83(1) 
xe2x80x83Na2B4O7+16Na+8H2+7SiO2xe2x86x924NaBH4+7Na2SiO3xe2x80x83xe2x80x83(2)
The primary energy cost of these processes stems from the requirement for a large amount of sodium metal, e.g., 4 moles of sodium per mole of sodium borohydride produced. Sodium metal is commercially produced by electrolysis of sodium chloride with an energy input equivalent to about 37,500 BTU (39,564 KJ) per pound of sodium borohydride produced. In contrast, the hydrogen energy stored in borohydride is about 10,752 BTU (11,341 KJ) of hydrogen per pound of sodium borohydride. The Schlesinger process and the Bayer process, therefore, do not provide a favorable energy balance, because the energy cost of using such large amounts of sodium in these reactions is high compared to the energy available from sodium borohydride as a fuel.
Furthermore, in view of the large quantities of borohydride needed for use, e.g., in the transportation industry, these processes would also produce large quantities of NaOCH3 or Na2SiO3 waste products. Since these byproducts are not reclaimed or reused, further energy and/or expense would need to be expended to separate and properly dispose of these materials.
Typical improvements of the prior art describe simple modifications of the two processes given in equations (1) and (2). As such, however, these improvements also suffer from the disadvantages stated above and do not provide any improved energy efficiency. It can be seen, therefore, that the widespread adoption of borohydride as a source of hydrogen would almost necessitate a recycle process that would allow the regeneration of borohydride from the borate byproduct. Thus, borohydride can be used as a fuel, and the resulting borate can then be recycled back to generate borohydride. However, such a process cannot rely on the same sodium stoichiometry shown in the current borohydride manufacture processes, e.g., the Schlesinger process of Equation (1) or the Bayer process of Equation (2).
The present invention provides processes for producing large quantities of borohydride compounds, which overcome the above-described deficiencies. In addition, the efficiencies of the processes of the present invention can be greatly enhanced over the typical processes for producing borohydride compounds.
In one embodiment of the present invention, a process is provided for producing borohydride compounds, which includes reacting the boron-containing compound BX3 with hydrogen to obtain diborane (B2H6) which, is turn, reacted with a Y-containing base selected from those represented by the formulae Y2O, Y2CO3 and YOH to obtain YBH4 and a YBO2, wherein Y is selected from the group consisting of the alkali metals, pseudo-alkali metals, alkaline earth metals, an ammonium ion, and quaternary amines of formula NR4+, wherein each R is independently selected from hydrogen and straight- or branched-chain C1-4 alkyl groups; and X is selected from the group consisting of halide ions, hydroxyl, alkyl or alkoxy groups, chalcogens, and chalcogenides.
In another embodiment of the present invention, a process is provided for producing borohydride compounds, which includes reacting a boron-containing compound of the formula BX3 with a Y-containing base of the formula YH to obtain YHBX3; and separately reacting BX3 with hydrogen to obtain diborane which is, in turn, reacted with YHBX3 to obtain YBH4 and BX3, wherein X and Y are as defined above.
In either of these embodiments, the Y-containing base of the formula Y2O and the boron-containing compound of the formula BX3 can be obtained by the following processes. The first process includes: (A) reacting a borate of the formula YBO2 with CO2 and H2O to obtain YHCO3, and borax; (B) heating YHCO3 to obtain Y2O, CO2 and H2O; (C) separately reacting borax with an acid to obtain boric acid B(OH)3 which is isolated and dehydrated to B2O3; (D) reacting the B2O3 with carbon and X2 to obtain BX3 and CO2. The second process includes: (I) reacting a borate of the formula YBO2 with CO2 and an alcohol to obtain YHCO3 and B(OR)3 wherein R is a lower alkyl group; (II) heating YHCO3 to obtain Y2O, CO2 and H2O; (III) reacting the B(OR)3 with H2O to obtain B(OH)3, which is dehydrated to form B2O3; and (IV) reacting the B2O3 with carbon and X2 to obtain BX3 and CO2. The Y-containing base compounds of the formula Y2CO3 can be obtained by replacing steps (B) and (II) with the following step (B2): converting the YHCO3 to Y2CO3, CO2 and H2O. Alternatively, the boron-containing compounds BX3 can be obtained by replacing steps (C) and (D) with one of the following steps: (Cl) where X is a halide, reacting borax with carbon and X2 to obtain BX3 and CO2; or (C2), where X is an alkoxy group, reacting borax with an alcohol to obtain BX3.
In still another embodiment of the present invention, a process is provided for producing borohydride compounds, which includes: (A) reacting a borate of the formula YBO2 with CO2 and H2O to obtain YHCO3 and a B2O3 compound; (B) heating the YHCO3 to obtain Y2O, CO2, and H2O; (C) reacting the B2O3 compound with an alcohol to obtain BX3; (D) reacting methane with the Y2O to obtain Y, carbon monoxide and H2; (E) reacting the Y with H2 to obtain YH; (F) reacting the BX3 with the YH to obtain YHBX3; (G) separately reacting BX3 with H2 to obtain B2H6 and HX; and (H) reacting the YHBX3 with B2H6 to obtain YBH4 and BX3 wherein Y and X are as defined above.